1. Field of the Invention
This invention relates to a lens control device suited for automatic focusing, and the like.
2. Description of the Prior Art
In lenses of the kind that are driven for purposes such as automatic focusing, zooming, and for similar effects, the range of lens movement is limited. Therefore, when such a lens reaches the limit of movement during a lens driving process, the arrival at the limit must be detected by some means. In a conventional arrangement, a limit switch is provided at the limit of lens motion in order to detect arrival at the limit. This method of providing such a mechanical switch, however, is not only disadvantageous in terms of its structural arrangement but also necessitates a troublesome adjustment of the limit position relative to the lens. It has been, therefore, difficult to arrange for precise limit detection.
In addition to this, it has been another problem in conventional lens structures that, in driving the lens for automatic focusing, it is not possible to adjust the defocusing or degree of focus shift as a function of unit time at a fixed rate. More specifically stated, even if the lens is driven at a constant driving speed, the degree of shift in the focus varies with the kind of the lens. For example, it varies when zoom lenses of different zoom ratios are used and it also becomes completely different between a telephoto application and a wide-angle application of a zoom lens. Therefore, even if the lens driving speed is made constant to obtain a constant shift in focus per unit time for focus adjustment in a telephoto application of a zoom lens, the extent of focus shift per unit time becomes slower when the zoom lens is used for a wide-angle application. Thus, focusing action becomes slower on wide-angle application than in telephoto application.